The Amazon rainforest, one of the planet's most precious and besieged natural resources, is even more fragile than realized.

If the planet warms even a moderate amount, a new study predicts that as much as 40 percent of it could be condemned to vanish by the end of the century.

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Chris Jones of the United Kingdom's Met Office and a group of researchers ran a computer simulation of Earth's climate that focused on how vegetation reacts to warming. They found that warming doesn't immediately kill off tropical trees -- it can take up to a century for the forests to respond fully.